The invention relates generally to the field of image capture, and more specifically to a method of exposure control in a camera having an extended dynamic range image sensing device.
Image sensing devices such as a charge-coupled device (CCD), are commonly found in such products as digital cameras, scanners, and video cameras. These image sensing devices have a limited dynamic range when compared to traditional photographic film products. A typical electronic image sensing device has a dynamic range of about 7 stops. This means that the exposure for a typical scene must be determined with a fair amount of accuracy in order to avoid clipping the resultant signal. By contrast, natural scenes often exhibit a dynamic range of 9 stops and higher. This is mainly a consequence of multiple light sources with widely varying intensities illuminating the scene objects. Specular highlights also contribute to the dynamic range of natural scenes.
Electronic sensors used to scan photographic film must also contend with a high dynamic range of signal intensities. U.S. Pat. No. 5,221,848 issued Jun. 22, 1993 to Milch entitled High Dynamic Range Film Digitizer and Method of Operating the Same discloses a method and apparatus designed to extend the dynamic range of an electronic image sensor. Aimed primarily for scanning photographic film, Milch teaches a method of a one pass film scanner using a charge-coupled device scanner having a plurality of linear arrays having the same spectral sensitivity. One of the arrays has a faster response to light than the other array. The information from the two arrays is then combined and digitized forming an extended dynamic range digital image.
Digital electronic cameras employ a single image sensor with a color filter array (CFA) to produce a sparsely sampled digital image. A typical color filter array pattern is disclosed in U.S. Pat. No. 3,971,065 issued Jul. 20, 1976 to Bayer entitled Color Imaging Array. Interpolation algorithms are employed to produce a full resolution color image from the sparsely sampled image. Digital cameras also need to record scenes having a high dynamic range. One way to obtain a high dynamic range image from a digital camera is to employ a high bit depth analog to digital converter in the camera. Another way is to employ an image sensor having interspersed fast and slow photosites as disclosed in copending U.S. Ser. No. 09/615,398 filed Jul. 13, 2000 by Gallagher et al., which is incorporated herein by reference. Also an important feature of digital cameras is the need to produce digital images with different spatial resolution. This is primarily due to the limited on board memory of the digital camera and due to the limitations in bandwidth for transferring the digital images to other devices sparsely sampled extended dynamic range digital images.
Digital cameras employ electronic image sensors, and have by virtue of the imaging application, a need to record a digital image with high dynamic range. Also an important feature of digital cameras is the need to control the exposure of light received by the electronic image sensor in a manner which makes maximal use of the dynamic range of the electronic image sensor. Conventional techniques for exposure control in digital cameras include using a separate photocell to control exposure, and using the signal from the image sensor in the camera to control exposure. The conventional exposure control techniques do not take into account benefits that may be achieved by the use of an image sensor having fast and slow photosites.
Therefore, there exists a need for electronic image sensors which are capable of recording images with extended dynamic range. Furthermore, there is a need for a method of exposure control which uses the unique attributes of a wide dynamic range electronic image sensor having fast and slow photosites.
The need is met according to the present invention by providing a method of exposure control in a camera employing a sparsely sampled extended dynamic range image sensing device that includes the steps of: providing a sparsely sampled extended dynamic range image sensing device having fast photosites with a predetermined response to light exposure interspersed with slow photosites with a slower response to the same light exposure; using the image sensor to produce a sparsely sampled high resolution digital image having fast pixel values produced by the fast photosites and slow pixel values produced by the slow photosites; calculating an exposure control value using only the slow pixel values from the a sparsely sampled extended dynamic range image; and using the exposure control value to modify the exposure to the image sensing device.
The invention has the advantage that exposure is controlled such that image information loss due to photosite saturation is minimized. The invention has the further advantage that the exposure control value is calculated in a computationally efficient manner by excluding fast pixel values from the computation. The invention has the further advantage that accurate exposure control is provided by using low resolution images derived from the sparsely sampled high resolution digital image.